1. Field of the Invention
This invention relates generally to multi-channel wireless communications systems. In particular, the present invention relates to a technique for reducing spurious and inter-modulation products through static non-linearity correction in a wireless broadband digital multi-carrier transceiver.
2. Background of the Invention
A conventional cellular phone system 100 is shown in FIG. 1. As illustrated in FIG. 1, the cellular phone system 100 includes a plurality of cells 110a, 110b, a mobile unit 120, a plurality of base station transceivers (BTS) 105a, 105b, communication lines 140, a mobile telecommunications switching office (MTSO) 130, an interface 150 and a switched telephone network 160.
The cellular phone system 100 has a fixed number of channel sets distributed among the BTS 105a, 105b serving a plurality of cells 110a, 110b arranged in a predetermined reusable pattern. The mobile unit 120, in a cell 110a or 110b, communicates with the BTS, 105a or 105b, respectively, via radio frequency (RF) means. The BTS 105a, 105b communicate with the MTSO 130 via communication lines 140. The MTSO 130 communicates with the switched telephone network 160 via the interface 150.
In the cellular phone system 100, the cell areas typically range from 1 to 300 square miles. The larger cells typically cover rural areas, while the smaller cells typically cover urban areas. Cell antenna sites utilizing the same channel sets are spaced by a sufficient distance to assure that co-channel interference is held to an acceptably low level.
The mobile unit 120 in a cell 110a has radiotelephone transceiver equipment which communicates with similar equipment in BTS 105a, 105b as the mobile unit 120 moves from cell to cell.
Each BTS 105a, 105b relays telephone signals between mobile units 120 and a mobile telecommunications switching office (MTSO) 130 by way of the communication lines 140.
The communication lines 140 between a cell site, 110a or 110b, and the MTSO 130, are typically T1 lines. The T1 lines carry separate voice grade circuits for each radio channel equipped at the cell site, and data circuits for switching and other control functions.
The MTSO 130 in FIG. 1 includes a switching network (not shown) for establishing call connections between the public switched telephone network 160 and mobile units 120 located in cell sites 110a, 110b and for switching call connections from one cell site to another. In addition, the MTSO 130 includes a dual access feeder (not shown) for use in switching a call connection from one cell site to another. Various handoff criteria are known in the art and utilize features such as delay ranging to indicate the distance of a mobile unit from a receiving cell site, triangulation, and received signal strength to indicate the potential desirability of a handoff. Also included in the MTSO 130 is a central processing unit (not shown) for processing data received from the cell sites and supervisory signals obtained from the switched telephone network 160 to control the operation of setting up and taking down call connections.
As the popularity of cellular systems grow, the performance of the cellular system needs to improve as more users subscribe to cellular systems. The performance of the BTS is one area of cellular systems that has been the focus of improvement.
A block diagram of a modern wireless broadband digital multi-carrier transceiver system (BTS) 200 is illustrated in FIG. 2. The BTS 200 includes a network interface module 205, a digital signal processing (DSP) module 210, a combiner module 215, a channelizer module 220, and a broadband RF transceiver 225.
The network interface module 205 provides an interface between the BTS 200 and a MTSO. In this particular example, the network interface module 205 provides ninety-two (92). 16 kbps subrate voice channels.
The network interface module 205 is also interfaced with the DSP module 210. The DSP module 210 provides for channel coding and modulation of thirteenxe2x80x94(13) kbps voice channel data from the network interface module 205. The DSP module 210 multiplexes eight (8) channels into a single baseband signal for upconversion and combining with other RF carriers by the combiner module 215. The DSP module 210 also provides the equalization, demodulation and channel decoding from received channels of RF carriers that have been downconverted to a baseband signal by the channelizer module 220.
The combiner module 215 provides for receiving the baseband RF carriers from the DSP module 210. Each RF carrier is filtered and upconverted to a unique intermediate frequency (IF). All of the RF carriers in a fivexe2x80x94(5) megaHertz (MHz) bandwidth are simultaneously combined into a single composite IF signal. This digital IF signal is then transferred to the broadband RF transceiver 225.
The channelizer module 220 provides for receiving a digital composite IF signal from the broadband RF transceiver 225. The digital composite IF signal consists of all twenty-five (25) of the 200-kiloHertz (kHz) RF carriers in a 5 MHz bandwidth. The channelizer module 220 provides for filtering and downconverting each RF carrier to a baseband signal for processing by the DSP module 210.
The broadband RF transceiver 225 provides for conversion of the digital signals for transmission to mobile users. The broadband RF transceiver 225 also provides for conversion of received analog signals to digital signals for processing by the BTS 200.
The broadband RF transceiver 225 includes an upconverter 230, two downconverters 235a, 235b, and a broadband converter 245.
Each downconverter, 235a or 235b, accepts at least a 5 MHz wide block of RF frequencies and downconverts to an IF center frequency, and then passed to the broadband converter module 245. The broadband converter module 245 digitizes the incoming analog signal and then passes the digital signal to the channelizer module 220.
The broadband converter module 245 also receives a digital broadband signal from the combiner module 215. The broadband converter module 245 converts the digital broadband signal into an analog IF signal. After filtering, the analog IF signal is upconverted to RF, and transmitted to a mobile user by the transmitter 230.
The broadband converter module 245 includes low pass analog filters 250a, 250b, a transmit digital half-band filter 255a, receive digital half-band filters 255b, 255c, a digital-to-analog converter (DAC) 260, and an analog-to-digital converter (ADC) 265a, 265b. 
The low pass filters 250a, 250b provide filtering for the incoming analog signals from a mobile user.
The transmit digital half-band filter 255a provides interpolation-by-two on the transmitted digital data from the combiner module 215.
The receive digital half-band filter 225b, 255c provides for decimation-by-two and filtering of the received digitized analog signals from a mobile user.
The DAC 260 provides for a conversion of digital signals to analog signals in preparation for transmission to a mobile user.
The ADC 265a, 265b provides for a conversion of received analog signals from a mobile user into digital signals to be processed by the BTS 200.
In using a conventional digital multi-carrier BTS, the ADC and/or DAC of the BTS must operate as linearly as possible in order to avoid adjacent channels and/or harmonic distortion in the transmitted signal. However, the operational characteristics of the ADC and/or DAC may hinder in achieving the goal of linear operation.
Although designed for linear operation, the typical ADC and/or DAC are not perfectly linear. This is due to imperfect transfer functions of the converters. The imperfection may be caused by errors in resistor ladders, which are used to establish the conversion thresholds in the ADC and/or DAC.
The imperfect transfer functions of the ADC and/or DAC introduce two type of errors: integral non-linearity (INL) and differential non-linearity (DNL). INL may be defined as the worst case deviation from the straight-line approximation of a given converter""s transfer function. DNL may be defined as the worst case deviation from an ideal step size between adjacent codes along the transfer curve. These two types of errors typically dominate the harmonic content of the spectrum. As the converter operates, the INL and DNL introduce spurs and intermodulation products, which restricts the spurious free dynamic range (SFDR) of the ADC and/or DAC.
SFDR is a measurable characteristic of the ADC and/or DAC that is very important in communications. SFDR is typically the usable dynamic range before spurious noise interferes or distorts the fundamental signal. In the cellular telephone environment, the bandwidth for the mobile users is typically shared. If a broadband transmitter sends spurious signals into other frequency bands used by other calls, the spurious signals can corrupt, interrupt, or obliterate the neighborhood frequency bands. Similarly, if a broadband receiver receives strong in-band interference, then spurious or intermodulation products can corrupt low-level receive signals. Thus, an ADC and/or a DAC with a wide SFDR would be preferable in order to ensure linear operation.
In addition to linear operation, it would be preferable that the ADC and/or DAC also have a high sampling rate. The high sampling rate is preferred due in large part to the Nyquist Sampling Theorem. The Nyquist Sampling Theorem requires that the sampling rate be at least twice as fast as the highest frequency component of the input signal. Violation of the Nyquist Sampling Theorem results in an effect known as aliasing where the sampled signal is not sufficiently represented and can not be restored. Typically, cellular systems operate over 5-15 MHz wide RF bands which demands a high sampling rate (e.g., 10-30 MHz) just to satisfy the minimum requirement of the Nyquist Sampling Theorem. Further, additional bandwidth surrounding the IF signal would be preferred for the purposes of analog anti-alias filtering, digital filtering, and decimation.
Thus, it would be desirable to have an ADC and/or DAC with a high SFDR and a high sampling rate. Some of today""s state-of-the-art ADC and/or DAC have the high sampling rate (e.g., 50 MHz). However, these ADC and/or DAC typically have only moderate SFDR (e.g., 70 dB) that may not meet some cellular system constraints.
In order to overcome these limitations, some solutions have been suggested. Laser trimming of the resistor ladders in the ADC and/or DAC is one suggestion. By using a laser to trim the resistor ladders, the performance of the ADC and/or DAC may be improved. However, laser trimming typically yields a 6 to 12 dB improvement in SFDR at a relatively high cost.
Another solution to improving the performance characteristics of the ADCs and/or DACs is to design with a greater number of bits. By increasing the number of bits, the performance of the ADC and/or DAC may improve by approximately 6 dB per bit. However, the drawback to this solution is that the ADCs and/or DACs typically require the use of expensive hybrid technologies. Moreover, the ADCs and/or DACs draw more power for a marginal increase in performance while typically operating at slower sample rates.
Out-of-band dithering may provide another solution for improving the performance characteristics ADCs and/or DACs. Out-of-band dithering randomizes the frequencies of spurious signals and spreads the frequencies onto the noise floor; however, this also raises the noise floor and does not provide the best SFDR improvement.
There is therefore a need for methods and apparatus to reduce static non-linearity errors and increase the SFDR for a wireless broadband digital multi-carrier transceiver system.
It is an object of the present invention to provide a method and apparatus for reducing static non-linearity errors in a broadband wireless digital multi-carrier transceiver system.
It is a further object of the present invention to provide a method and apparatus for increasing the SFDR for a broadband wireless digital multi-carrier transceiver system.
In accordance with the principles of the present invention, static differential non-linearity errors of the digital to analog (D/A) converters and the analog to digital (A/D) converters in a wireless broadband transceiver system may be corrected at full operational speed via a real-time digital look-up table. Initially, the values for the digital look-up table are determined prior to full operational use of the wireless broadband transceiver system. The one set of values represents inverse response of the transfer function of the D/A converter. The other sets of values represents the inverse response of the transfer function for each A/D converter. The values for the D/A and A/D converters are stored in a non-volatile memory.
During initialization of the wireless broadband transceiver system, the values of the inverse response of the transfer function for the D/A converter are transferred from the non-volatile memory to a high-speed memory. The values of the inverse response of the transfer function for each A/D converter are also transferred to another set of high-speed memory. Thus, the high-speed memories form a digital look-up table.
For the transmission of a signal, the signal is used as an address into the high-speed memory to output a corrected value for the D/A converter. For example, a 16-bit digital path having 96 dB of SFDR resulting from digital signal processing, e.g., inverse Fast Fourier Transform (FFT), may be maintained through a lower-resolution DAC (e.g., 12 bit), because of the correction prior to the DAC.
For a received signal, each output from the lower resolution (e.g., 12-bit) ADC is used as an address into each high-speed memory. The 16-bit output from each 12-bit address of the high-speed memory is a corrected value for each respective ADC. For example, a 16-bit digital signal processing operation, e.g., an FFT, can utilize the improve SFDR which results from a 16-bit linearization following each lower-resolution ADC.
By using real time correction of non-linear errors by digital look-up table, the static differential non-linearity errors of the D/A and A/D converters, which are masked into the integrated circuits themselves, are effectively undone through the digital correction table at the converters"" sampling rate. This allows for operation of broadband cellular base station transceiver equipment at a much-improved spurious free dynamic range.